A thin film transistor including a semiconductor film formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since there is limitation on the allowable temperature limit of a substrate in manufacture of a thin film transistor, a thin film transistor in which amorphous silicon that can be deposited at a relatively low temperature, polysilicon that can be obtained by crystallization with use of laser light or a catalytic element, or the like is included in an active layer is mainly used for a semiconductor display device.
In recent years, a metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high electron mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. The metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are known (Patent Documents 1 and 2).